Pass the XHR object in callback

This is required because the Etag is returned in header. If we want to read
it, we need access to the XHR object. This would have been much easier if
Amazon passed the ETag in response XML as well.

This might look a bit inconsistent. (If `parseXML` is `true`, we get parsed
response XML. Otherwise, we get the XHR object.) But it's done this way so
as to not break S3Ajax API.